1. Field of the Invention
The present invention relates to the production of hydrocarbons from a borehole. More particularly, the invention relates to a method and apparatus for perforating and fracturing a formation surrounding a borehole.
2. Description of Related Art
Techniques for perforating and fracturing a formation surrounding a borehole are known in the art. The most common technique for perforating and fracturing a formation to stimulate production includes the steps of: 1) penetrating a production zone with a projectile, such as a shaped charge; and 2) hydraulically pressurizing the borehole to expand or propagate the fractures initiated by the shaped charge.
Modern shaped charges are widely used for both military and commercial applications. Although the main operation is remarkably similar in both applications, there are at least two significant differences in the devices actually employed. One difference is cost. Military applications generally demand much higher performance and, in particular, high reproducibility. This, in turn, requires the liner portion of the shaped charge to be forged and precision machined.
In the commercial use of the shaped charge in oil or gas well stimulation, the jet from the shaped charge is employed to create a flow path from the reservoir to the wellbore. In this application, a large number of perforators is inserted into the wellbore in what is called a gun. Although there are three basic types of guns, perhaps the most common is the casing gun, which can be run into the well on a wireline or conveyed by tubing. The charges are contained in a steel tube, protected from impact and from the well fluids, and are arranged so that they face radially outward from the vertical axis of the carrier. In these devices, the liners are pressed using powder metal technology and are relatively less expensive than those used in typical military uses, e.g., missile warheads.
Another factor that distinguishes commercial shaped charges from those used in weapons is standoff, i.e., the distance from the liner base to the target (usually measured in charge diameters). The penetrating effectiveness of a shaped charge jet is markedly enhanced by standoff. The reason is that shaped charge jets normally are formed with a high axial velocity gradient, the tip moving at speeds of 6-10 km/s. The standoff distance allows the jet to stretch or elongate before encountering the target and, to first order, the depth of penetration is directly proportional to the length of the penetrator. There is an optimum standoff. If the distance to the target is too great, the penetration can be much less than if there were no standoff. This occurs because the jet can only stretch a given amount before breaking; once broken the particles are easily deflected by small perturbations and no longer produce a coherent, unidirectional penetrator. With optimal standoff, typically 6-8 charge diameters (CD), the penetration can be enhanced by 50% or more, relative to that achieved with zero standoff. Commercial perforators, however, are rarely able to operate at more than 1 CD because they must fit inside the casing gun which, in turn, must fit inside the casing.
Techniques to increase the efficiency of hydrocarbon production in the borehole utilizing guns and pressurizing sections of the borehole have been described in the recent past. For example, Petitjean, U.S. Pat. No. 5,355,802, describes a method and apparatus for firing a shaped charge through a gas zone of propellant combustion gases. A propellant is ignited downhole, releasing gas into the borehole to pressurize a portion of the borehole. The firing of the shaped charges is delayed until the pressure level is significantly above the breakdown pressure of the formation, but still below that of the casing. Although such a technique can improve the penetration effects of shaped charge perforators, a need still exists to continually improve penetration of a reservoir surrounding a borehole.